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</div><hr /><div id="intro.chdb" class="preface">
  <h1 class="title">简介</h1>
  <p class="para">
   CHDB (constant hash database) is a fast key-value database for constant data,
   realized by using a memory-mapped file and thus providing the following
   functionalities:
   <ul class="itemizedlist">
    <li class="listitem">
     <p class="para">
      Extremely fast initial load, regardless of the size of the database.
     </p>
    </li>
    <li class="listitem">
     <p class="para">
      Only the pages of the file which are actually used are loaded from the
      disk.
     </p>
    </li>
    <li class="listitem">
     <p class="para">
      Once a page is loaded it is shared across multiple processes.
     </p>
    </li>
    <li class="listitem">
     <p class="para">
      Loaded pages are cached across multiple requests and even process recycling.
     </p>
    </li>
   </ul>
  </p>
  <p class="para">
   A typical use of CHDB is as a faster alternative to defining many PHP
   constants.
  </p>
  <p class="para">
   CHDB is internally implemented as a hash-table using a
   <a href="http://en.wikipedia.org/wiki/Perfect_hash_function" class="link external">&raquo;&nbsp;perfect hashing</a>
   function, thus guaranteeing worst case O(1) lookup time.
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